Historical contingency and an herbivorous calamity

This post was chosen as an Editor's Selection for ResearchBlogging.org

A while ago I asked, “What the hell was Australopithecus boisei doing?” To recap: there’s this weird side branch of human evolution that was dubbed “Australopithecus boisei” and lived in Eastern Africa from around 2.3 – 1.4 million years ago. They lived right alongside our ancestors, early Homo. If you think human diversity is remarkable today, you’d be totally blown away by the diversity of the early Pleistocene. Since 1959 when A. boisei (then Zinjanthropus boisei) was first discovered, people noticed its massive molar and premolar teeth, thick and powerful jaws, and muscle markings indicative of diabolical chewing power. ‘Probably subsisted on a diet of low-quality, hard to chew foods,’ people reasoned.

But a few years ago, this picture changed: evidence from toothwear and the chemical composition of teeth suggested A. boisei was actually eating grass or sedges (see the referred post or a nice recent review by Julia Lee-Thorp for more info). Such a diet is totally at odds with what people had hypothesized based on the size of the chewing muscles and teeth.

Colobus molars, good for shearing apart leaves. (image: http://bit.ly/xefm6t)

I was discussing this last point with a colleague the other day, who could not believe A. boisei ate grasses or the like: Many animals known to eat grass or leaves tend have molars with high crowns with slicing edges for shearing apart a mouthful of vegetation (above), but A. boisei molars are large and low-cusped, becoming fairly flat with wear (below).

Australopithecus boisei specimen KNM-ER 15930 (Leakey & Walker 1988, Figure 8)
But, it occurred to me, maybe high-crowned, shearing molars simply were not an ‘option’ in the evolution of Australopithecus boisei. Natural selection is a powerful force of evolution, but it is limited because it can work only with existing variation: it does the best it can with what it’s got. The earliest surefire hominins, Australopithecus anamensis and afarensis, certainly did not have ‘cresty’ molars with pointy cusps, and neither did many late Miocene apes, for that matter. Rather, the ancestors of A. boisei had fairly low bulbous molar cusps, and that’s some serious evolutionary baggage for a hominid hoping to corner the grass and sedge market.
So we can draw up the following hypothesis for the evolution of A. boisei: as the early members of the species moved into a niche of eating grass/sedges, rather than evolve cresty teeth, they increased the size and enamel thickness of their ancestors’ molars to better-withstand their diet. Perhaps this was the ‘easiest’ solution to adapting teeth to a crappy diet (maybe some developmental constraint?). Or perhaps there’s another, yet unidentified food responsible for the species’ curiously high-C4 diet … who knows? Nota bene: this isn’t necessarily what I think happened, it’s just a hypothesis consistent with current evidence about A. boisei‘s anatomy and diet.
If Life on Earth has taught us anything, it’s that there are many ways to do the same thing. What’s more, evolution is highly constrained by pre-existing biology and historical circumstance. Australopithecus boisei may have been ‘a victim of its times,’ forced into an herbivorous niche for which it was ill-equipped.
Leakey RE, & Walker A (1988). New Australopithecus boisei specimens from east and west Lake Turkana, Kenya. American Journal of Physical Anthropology, 76 (1), 1-24 PMID: 3136654
Lee-Thorp, J. (2011). The demise of “Nutcracker Man” Proceedings of the National Academy of Sciences, 108 (23), 9319-9320 DOI: 10.1073/pnas.1105808108
*Edited 07 Nov 2015